[docs] Clarify semantics of ordered fadd/fmul reductions.

[spatel]

llvm/docs/LangRef.rst

15135

"That is, the operation begins with the start value and performs..." ?

15204–15205

Add the corresponding text/example for fmul too?

[aemerson]

llvm/docs/LangRef.rst

15135

Maybe "the reduction begins with" would be clearer.

15204–15205

Will do.

[RKSimon]

LGTM - thank you.

[spatel]

llvm/docs/LangRef.rst

15135

Grammar nit: remove comma from "value,"
Similar in the fmul text.

llvm/docs/LangRef.rst

 
 The '``llvm.experimental.vector.reduce.v2.fadd.*``' intrinsics do a floating-point
 ``ADD`` reduction of a vector, returning the result as a scalar. The return type
 matches the element-type of the vector input.
 
 If the intrinsic call has the 'reassoc' or 'fast' flags set, then the
 reduction will not preserve the associativity of an equivalent scalarized
 counterpart. Otherwise the reduction will be *ordered*, thus implying that
-the operation respects the associativity of a scalarized reduction.
+the operation respects the associativity of a scalarized reduction. That is, the
+reduction begins with the start value and performs an fadd operation with consecutively
+increasing vector element indices. See the following pseudocode:
+
+::
+
+    float ordered_fadd(start_value, input_vector)
+      result = start_value
+      for i = 0 to length(input_vector)
+        result = result + input_vector[i]
+      return result
 
 
 Arguments:
 """"""""""
 The first argument to this intrinsic is a scalar start value for the reduction.
 The type of the start value matches the element-type of the vector input.
 The second argument must be a vector of floating-point values.
 

 """""""""
 
 The '``llvm.experimental.vector.reduce.v2.fmul.*``' intrinsics do a floating-point
 ``MUL`` reduction of a vector, returning the result as a scalar. The return type
 matches the element-type of the vector input.
 
 If the intrinsic call has the 'reassoc' or 'fast' flags set, then the
 reduction will not preserve the associativity of an equivalent scalarized
 counterpart. Otherwise the reduction will be *ordered*, thus implying that
-the operation respects the associativity of a scalarized reduction.
+the operation respects the associativity of a scalarized reduction. That is, the
+reduction begins with the start value and performs an fmul operation with consecutively
+increasing vector element indices. See the following pseudocode:
+
+::
+
+    float ordered_fmul(start_value, input_vector)
+      result = start_value
+      for i = 0 to length(input_vector)
+        result = result * input_vector[i]
+      return result
 
 
 Arguments:
 """"""""""
 The first argument to this intrinsic is a scalar start value for the reduction.
 The type of the start value matches the element-type of the vector input.
 The second argument must be a vector of floating-point values.